Q: What is happening to my plum tree? It finished bearing plums in July and now the trunk is a sea of sap! Should I do anything?
A: I think it’s gummosis, a condition that’s very difficult to control on a single tree. Most often it is caused by bacterial or fungal canker. The peach/cherry/plum tree becomes infected through a wound or a poor pruning cut. The tree tries to defend itself by exuding gum around the damaged area.
Because the infection is under the bark, no spray will control the canker. You have to help the tree be as healthy as possible in order to fight the canker itself.
To that end, in the absence of a soil test, sprinkle 1 cup of garden lime per inch of trunk thickness under the branch canopy every year for three years. This will raise the soil pH, which peaches/cherries/plums appreciate.
Water the tree appropriately in summer so the roots are not damaged by drought.
Gummosis can also be caused by an attack by borers. In my experience this is rare, but examine the gum to see if any wood particles are in it. If there are several small pieces of sawdust in the gum, call your local Extension office (1-800-ASKUGA-1) for advice.
see also Gummosis